1. Field of the Invention
This invention generally relates to a method for manufacturing a rail for use in a linear motion rolling contact guide unit, and, in particular, to a method for manufacturing such a rail at extremely high accuracy and precision.
2. Description of the Prior Art
A linear motion guide unit is well known in the art, and it generally includes a rail, a slider and a plurality of rolling members interposed between the rail and the slider for providing a rolling contact between the rail and the slider. There are basically two types of such a linear motion guide unit, i.e., the infinite stroke type and the finite stroke type. Such a typical prior art linear motion guide unit is illustrated in FIG. 7. As shown, the illustrated linear motion guide unit includes a rail 1 extending straight over a desired length, a slider 3 slidably mounted on the rail 1 in a straddling manner, and a plurality of balls 2 as rolling members interposed between the rail 1 and the slider 3. Use may also be made of rollers as rolling members in place of balls 2.
In the structure shown in FIG. 7, the rail 1 is elongated in shape and has a generally square or rectangular cross section so that it has a pair of opposite side surfaces 4, each of which is formed with an inner guide groove 5 extending in parallel with the longitudinal axis of the rail 1. Thus, the structure shown in FIG. 7 provides a two guide channel type linear motion guide unit. Alternatively, as shown in FIG. 8, another inner guide groove 7 may be formed in each of the opposite side surfaces, or at the top thereof, of the rail 1, in which case a four guide channel type linear motion guide unit is provided.
As shown in FIG. 7, the rail 1 is provided with a plurality of mounting holes 19 through which bolts or the like may be inserted to have the rail 1 fixedly mounted on a desired object. On the other hand, the slider 3 typically has a U-shaped cross section, including a horizontal section and a pair of side sections depending from the opposite sides of the horizontal section, so that the slider 3 is slidably mounted on the rail 1 in a straddling manner. The slider 3 is typically provided with a pair of endless circulating paths, including a load path section 6, a return path section and a pair of curved connecting path sections connecting the corresponding ends of the load and return path sections as well known in the art. An outer guide groove is formed in a surface of each of the side sections of the slider in an opposed relationship with a corresponding one of the inner guide grooves 5 of the rail to thereby define a load path section 6 as a guide channel between the associated pair of inner and outer guide grooves.
A plurality of rolling members or balls 2 in the illustrated example are provided in each of the endless circulating paths so that the balls 2 in the load path sections 6 provide a rolling contact between the rail 1 and the slider 3 and therefore the slider 3 may move relative to the rail 1 as long as the rail 1 extends.
When the slider 3 is detached from the rail 1 for some reason, such as maintenance or repair, mostly those balls 2 which are located in the load path sections 6 fall off. If this happens, it would be extremely difficult to put all of the balls 2 back in position when the slider 3 is mounted on the rail 1 once again. Thus, in order to prevent the balls 2 from falling off when the slider 3 is removed from the rail 1, a ball retaining member 8 extending generally along the load path section 6 is provided with its both ends fixedly attached to the slider 3 so as to prevent the balls 2 from falling off when the slider 3 is detached from the rail 1.
Such a ball retaining member 8 is integral with and thus moves with the slider 3 and thus it must be located not to scrub against the rail 1 when the slider 3 moves relative to the rail 1. For this reason, a relief trench 9 is provided at the bottom of each of the inner guide grooves 5 extending in parallel with the longitudinal axis of the rail 1 to receive therein a portion of the ball retaining member 8 without contact. In the prior art, such a relief trench 9 is typically either square or rectangular in shape. Incidentally, in the structure shown in FIG. 8, the additional guide groove 7 is not provided with a relief trench; however, as well known in the art, a plate-shaped projection is typically provided on the slider 3 to project therefrom toward the balls 2 to retain the balls 2 in position, though such a projection is not shown specifically.
Now, a typical prior art method for manufacturing the rail 1 will be described with reference to FIGS. 4 through 6.
(a) As shown in FIG. 4, a rail intermediate product 10 having a shape indicated by the two-dotted line is formed from an alloy steel material or the like by drawing. PA1 (b) After drawing, a relief trench 9 for receiving therein a corresponding ball retaining member 8 is formed at the bottom of each of inner guide grooves 11 by milling as shown in FIG. 5. PA1 (c) Then, a guide surface 12 of the inner guide groove 11 is hardened by induction hardening. PA1 (d) Then, after removing distortions due to heat treatment, selected portions of the rail intermediate product 10, including a top surface portion 13, a guide groove portion 11, a guide surface portion 12, a side surface portion 14 and a bottom surface portion 15, as indicated by the fat lines in FIG. 6, are ground to thereby finish the top surface 16, rail guide groove 5, side surface 17, and bottom surface 18 to a desired shape and accuracy, as shown by the solid line in FIG. 4.
Additional steps, such as a step for providing the mounting hole 19, are also carried out, but such additional steps are omitted since they do not form a part of the present invention. In FIG. 4, a contact relationship between the finished guide groove 5 and an associated ball 2 is illustrated. It is to be noted that a side recess 20 remains unprocessed after drawing as indicated by the dotted line in FIG. 6.
In accordance with the prior art, the relief trench 9 is formed by milling irrespective of the length of the rail 1 to be manufactured. Thus, if the rail 1 is relatively long, there may be produced a larger deformation after heat treatment so that there arise fluctuations in the position of the relief trench 9. Thus, after grinding, when the rail 1 and the slider 3 are assembled, there arises a case in which the ball retaining member 8 is in contact with the relief trench 9 of the rail 1. In order to avoid such an inconvenience, the width of the relief trench 9 must be made significantly larger as compared with the size of the ball retaining member 8. It is disadvantageous to set the width of the relief trench 9 larger since the effective ball bearing portion of the guide groove 5 is reduced and the rigidity of the guide groove 5 is also reduced.
As shown in FIG. 7, an end seal block 21 is provided at each end of the slider 3, and the end seal block 21 has a sliding end 22 which is in sliding contact with the top surface 16 and side surface 4 of the rail 1 so that a sealing characteristic between the rail 1 and the slider 3 during a relative motion between the rail 1 and the slider 3 is maintained at a predetermined level to thereby prevent any undesired foreign material from entering into the gap between the rail 1 and the slider 3. However, if there are significant fluctuations in the position of relief trench 9, the sealing characteristic of the end seal 21 may be impaired so that undesired foreign material may be allowed to enter in the gap between the rail 1 and the slider 3, which is particularly undesirable.
In addition, since the processing of the relief trench 9 and the processing of the guide groove 5 are carried out separately and before and after heat treatment in the prior art process, an absolute shift in position between the relief trench 9 and the guide groove 5 tends to be amplified, which can also be a cause of a sliding contact between the ball retaining member 8 and the relief trench 9. Furthermore, since the side recess 20 of the rail 1 remains unprocessed after drawing, a bottom surface seal 24 of the slider 3 cannot be brought into sliding contact with the side recess 20, and, thus, the sealing characteristic between the rail 1 and the slider 3 remains incomplete in this respect also.